Many of the existing cellular systems in the United States are analog-based systems. Because of significant increases in traffic capacity over the same frequency spectrum currently used in analog systems, lower cost operation, and additional mobile telephone features, such as battery-saving capabilities, a transition is being made from analog to digital communication in cellular telephone systems. That transition from analog to digital communication within the same frequency band is often achieved in a first step by replacing each of the fixed analog traffic channels with three digital traffic channels and in a second step by introducing a digital control channel, while maintaining as much as possible the same base station sites, cell sizes, and frequency plans designed for analog systems.
Despite the advantages purely digital or digital only mobile telephones have over their counterparts using analog control channels, many subscribers may be unwilling to discard their mobile telephones using analog control channels in order to purchase a purely digital mobile telephone. In addition, in many rural areas of the country, mobile telephones having only digital capabilities will not be practical because only analog control and traffic channels may be installed. On the other hand, while mobile telephones having analog control channels may be used anywhere in the country, they do not benefit from the new features intended for the digital only mobile telephones. It is therefore desirable to have a method of introducing the advantages attendant to purely digital cellular mobile telephones also to the mobile telephones using analog control channels.
In cellular systems using analog control channels, generally only one set of analog control channels is used. However, a second set of analog control channels, including dedicated control channels (DCC), paging channels (PC) and access channels (AC), may be used for setting up calls, informing the land system about the location of mobile stations, and for informing the mobile stations about the parameters of the land system. All mobile stations scan and read one of the dedicated control channels under certain specified conditions, for example, when the mobile stations become active, after a certain time period has elapsed, when ordered, or when the mobile station enters a new geographical part of the cellular system. A description of a cellular telephone system that incorporates a second set of analog control channels is set forth in U.S. patent application Ser. No. 514,540, filed Apr. 26, 1990, now U.S. Pat. No. 5,119,397 and assigned to the same assignee. The text of this patent is incorporated herein by reference.
While there has been little progress with respect to a completely integrated cellular telephone system with both analog and digital traffic and control channels, some paging systems have attempted to combine analog and digital capabilities. For example, U.S. Pat. No. 4,857,915 to Andros et al discloses a paging system which is compatible with transmissions from analog or digital paging transmitters, i.e. short, digital or analog voice messages may be transmitted. A paging receiver monitors an appropriate communication channel for its identification code. Starting with the least significant bits first, the paging receiver compares incoming digits on the particular communication channel with its own identification code. As soon as a mismatch occurs, the paging receiver turns itself off in order to conserve power. If the paging receiver determines that a match has been made by the received digital code with the paging receiver identification code, the paging receiver displays the place of origin of the page to the subscriber.
While Andros describes reception of pages from either analog or digital transmitters, Andros does not incorporate the features of a digital cellular telephone system with an existing analog cellular telephone system. More particularly, Andros does not disclose a cellular telephone system in which a mobile telephone may select either a digital or an analog control channel.